Senescence in Podospora anserina is maternally inherited and some races senesce more rapidly than others. During senescence, certain regions of the mitochondrial genome, termed senDNAs, amplify and suppress the replication of the non-senescent genome. Alpha senDNA, the first discovered, occurs most frequently and is found even in young cultures of race A, the most rapidly senescing race. DNA sequence analysis has shown that Alpha senDNA is a class II intron of the COI gene. Other senDNAs, for example Beta and Epsilon senDNA do not contain class II introns; Epsilon senDNA does contain class I introns in the URFI gene, one of which is self-splicing. Others have shown that certain class II introns, including Alpha senDNA contain putative amino acid sequences homologous to reverse transcriptase. Race A but not race s has a second class II intron close to Alpha senDNA in the COI gene which also has homology to reverse transcriptase. Our general objective is to relate the amplification of these senDNAs to senescence. We have two specific aims. 1. Investigate the Senescent Phenotype. Utilizing either outgrowth from senescent mycelia or specific mutagens, we have isolated certain phenotypes exhibiting altered growth characteristics. Restriction enzyme analysis has shown that gene rearrangements occur. We now wish to study the general nature of these mitochondrial gene rearrangements using specific probes and relate them to the senescent phenotype. 2. Study the Molecular Biology of Senescence. Race A has two class II introns in the COI gene whereas race s has but one. Are there other class II introns encoding reverse transcriptase? Using consensus sequences of class II introns will enable us to identify circular transcripts as well as their location on the genome. DNA sequence analysis will identify the reverse transcriptase homologue. We also wish to localize the reverse transcriptase activity found in older cultures. In no other system is there such a clearly developed link between molecular events and senescence. Gene rearrangements play a critical role in many developmental processes and the connection with reverse transcriptase provides significant relevance with many health related areas.
